How can Jesus be man and God at all times, when he was already existing

And now, Father, glorify me in your presence with the glory I had with you before the world began. John 17:5.

Before the world began, there was no man. So how could he acquire a human body from Mary when he wasn't born until about 0 Anno domini (A.D.)?

But Jesus was already existing and had glory with the Father even before the world began. John 17:5. But Jesus did not have a human body then. The creation of man did not occur until after the world was first created.